Mexico's announcement comes a day after it declared that no such incident occurred. Migrants arrested during a roundup allege that gunmen seized the group on Dec. 16 in Chiapas.

Reporting from Mexico City — Mexican officials said Wednesday that they are investigating the reported mass kidnapping of 50 Central American migrants, a day after declaring that no such incident took place.

The turnabout could head off possible diplomatic frictions. The National Institute of Migration said Commissioner Salvador Beltran del Rio was in touch with representatives of El Salvador and Honduras, which have drawn attention to the alleged Dec. 16 kidnapping in the southern state of Oaxaca.

Mexico's human rights commission opened its own investigation, saying it had heard from 18 migrants who said they witnessed the abductions.

Kidnappers have long preyed upon migrants traversing Mexico on their way to the United States, prompting charges that Mexico doesn't provide adequate security. The issue has grown more politically sensitive since the August massacre of 72 Central and South American migrants in northern Mexico.

Mexican officials vowed to redouble efforts against armed groups that kidnap migrants to extort money or recruit them for drug-trafficking activities.

Authorities regularly find large groups of migrants in safe houses along smuggling corridors, mainly in southern Mexico. Last month, officials announced the rescue of more than 100 people, mostly from Central America, who were being held in the southern state of Chiapas.

In the Oaxaca case, Central American migrants arrested during a roundup by Mexican authorities reported that 50 others had been seized by gunmen who stopped the train on which they were riding. Central American migrants often make their way north atop freight trains.

A Honduran migrant told radio host Carmen Aristegui that he and others fled after gunmen halted the train. "We heard the shots and had to run," he said.

El Salvador's government said it believes the migrants' account. But Mexican immigration officials said Tuesday afternoon that "no evidence exists of the train having been blocked or held up by any group."

But by Wednesday, the immigration agency said it was interviewing migrants from El Salvador, Honduras and Guatemala. Beltran said Mexican officials switched course after a Oaxaca priest, Alejandro Solalinde, contacted them with 19 witnesses willing to testify.

In San Salvador, Juan Jose Garcia, El Salvador's vice minister for citizens abroad, applauded the shift. "There is no question that we, the government of El Salvador, are onto the truth of the facts," he said in a brief interview.

A survivor said the migrants were seized by gunmen from the Zetas drug gang. Mexican officials said the migrants were killed for refusing to join the group, but some of the victims' relatives said they had received phone calls demanding money.